WO2003027994A2

WO2003027994A2 - Display system comprising a video processor

Info

Publication number: WO2003027994A2
Application number: PCT/IB2002/003967
Authority: WO
Inventors: William G. Miller; Mark Laramie; Gregg W. Keck
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2001-09-26
Filing date: 2002-09-26
Publication date: 2003-04-03
Anticipated expiration: 2004-03-26
Also published as: US6545717B1; US20030057838A1; JP2005504472A; KR20040037166A; EP1446789A2; WO2003027994A3

Abstract

A display system (10) which includes a cathode ray tube (CRT) feedback current replacement circuit (20) to simulate the cathode feedback current directly from the from the red, green and blue output biases of the drive signals of the video processor (13). Furthermore, the display system (10) applies vertical blanking to the driving stage (16) when the biases are applied so that these biases will not be visible during a vertical underscanned condition. Shifting the generation of the cathode feedback current prior to the application of the blanking prevents any significant distortion, modification or skewing of the cathode feedback current sensed by the AKB sensing circuitry (19) of the video processor (13).

Description

FIELD OF THE INVENTION

This invention relates to cathode ray tubes (CRT) or the like display systems, and more particularly to a display system having automatic CRT black level stabilization, also called auto kinescope bias (AKB) applying a feedback loop. The invention also relates to a method of blanking red, green and blue output biases of drive signals.

BACKGROUND OF THE INVENTION

Automatic cathode ray tube (CRT) black level stabilization is conventionally achieved by inserting bias voltages in the drive signals near the video black level during the vertical blanking interval. In a feedback system, relative DC biases of the drive signals are adjusted to achieve equal CRT red, green and blue (RGB) cathode currents near the video black level. Normally, these bias voltages are in the top overscanned part of the display and are not visible. However, in display modes in which the vertical deflection is underscanned, such as in the case where a 16:9 aspect ratio picture is displayed on a 4:3 display, these bias voltages become visible and distracting.

SUMMARY OF THE INVENTION

The present invention contemplates a display system which prevents the bias voltages from being visible. The invention is defined by the independent claims. The dependent claims define advantageous embodiments.

According to a first aspect of the present invention there is provided a display system as defined in the opening paragraph, comprising: a video processor producing red, green and blue drive signals having respectively a red output bias, a green output bias and a blue output bias corresponding to a respective black level of the respective drive signals; a cathode ray tube (CRT) having red, green and blue cathodes; a driving stage coupled to receive the red, green and blue output biases for driving the red, green and blue cathodes; a CRT feedback current replacement circuit which receives the red, green and blue output biases, which is coupled to the driving stage to generate cathode feedback replacement currents corresponding to the respective black levels , and to supply the replacement cathode feedback currents to the video processor; and blanking circuitry coupled to the driving stage for generating selectively a vertical blanking signal for blanking outputs of the driving stage, when the cathode feedback replacement currents are generated.

The system uses a replacement circuit, which also can be termed as a simulation circuit, to generate feedback directly from the drive signals of the video processor. In this way the feedback loop for stabilizing the black level of the cathode ray tube remains operating without requiring feedback from the driving stage.

So, vertical blanking can be applied to the driving stage to ensure that the CRT is completely cut off, despite the presence of the output biases at the inputs of the driving stage. So, for example, in case of vertical underscan, visibility of the output biases is avoided. This feature of using the feedback current replacement circuit in combination with the vertical blanking during the presence of the output biases does not have to be permanently activated when the display system is in an operation. Only if the display system is in an operation mode resulting in vertical underscanning it is useful to activate the feature.

So, the present invention contemplates a dfsplay system which has a means for selectively replacing AKB or, in other words, not using AKB and substituting an alternative cathode feedback current source such as during a vertical underscanned condition.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and illustrated with reference to the drawings, in which

Fig. 1 illustrates a schematic diagram of a conventional display system; Fig. 2 illustrates a schematic diagram of a display system in accordance of the present invention; and

Fig. 3 illustrates an alternate embodiment of the schematic diagram of a display system in accordance of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to Fig. 1, a schematic diagram of a conventional display system 1 with auto kinescope bias (AKB) is shown. The display system 1 includes a video processor integrated circuit (IC) 3 which monitors the current feedback on line 2, fed to the current feedback input terminal 5 during reference pulse intervals, and adjusts the red output bias 4R, the green output bias 4G, and the blue output bias 4B to maintain equal cathode currents. The integrated circuit 3 receives a supply voltage Ncc. The display system 1 further includes a CRT driving stage 6 having a plurality of CRT drive amplifiers 6a, 6b, 6c which are operational amplifiers. Each of the CRT drive amplifiers 6a, 6b, 6c have three outputs on lines 8_ls 8₂ and 8₃. The output on line 8_t of a respective CRT drive amplifier 6a, 6b, 6c drives a respective red, green and blue CRT's cathode RC, GC, and BC and sinks current. The output on line 8₂ of a respective CRT drive amplifier 6a, 6b, 6c supplies the cathode feedback current to the AKB sensing circuitry 9 of the video processor integrated circuit (IC) 3 on line 2. The output on line 8₃ of a respective CRT drive amplifier 6a, 6b, 6c is coupled to a first terminal of a respective one of the feedback resistors R7, R8, R9.

The CRT drive amplifiers 6a, 6b, 6c each have a resistor or impedance R4, R5 and R6, respectively, coupled to a first input terminal of a respective CRT drive amplifier 6a, 6b, 6c. Moreover, a second terminal of the feedback resistors R7, R8 and R9 is coupled to the respective first input terminal of its respective CRT drive amplifier 6a, 6b, 6c. Furthermore, each of the CRT drive amplifiers 6a, 6b, 6c have a second input terminal which receives on line 7 a vertical blanking signal having reference pulses during the reference pulse intervals. Referring now to Fig. 2, the display system' of the present invention is generally referenced by the numeral 10. The display system 10 in general eliminates visible bias voltages in the drive signals, when the vertical deflection is underscanned by (1) applying blanking (a vertical blanking signal) to prevent the bias voltages from being visible; and (2) replacing the conventional cathode feedback current from the outputs of the driving stage 16 with a cathode feedback replacement current from a CRT feedback current replacement circuit 20 positioned prior to the driving stage 16. The replacement current simulates the conventional cathode feedback current, so it can also be called the simulated cathode feedback current. The CRT feedback current replacement circuit 20 provides feedback from a position prior to the driving stage 16 so that a vertical blanking signal, without the reference pulses, on line 17, can be applied to the driving stage 16 without significantly distorting, modifying or skewing the cathode feedback current. This vertical blanking signal is supplied by a blanking circuit 17_ls and can be applied selectively for example only in case of vertical underscanning. The blanking circuit can be a separate circuit or a part of an other circuit. In the exemplary embodiment, the cathode feedback replacement current is simulated directly from the respective output bias of the red 14R, the green 14G and the blue drive signals 14B supplied by the video processor integrated circuit (IC) 13.

The CRT feedback current replacement circuit 20 includes a current mirror 22, a red transistor Ql having its base coupled to the red drive signal 14R, a green transistor Q2 having its base coupled to the green drive signal 14G and a blue transistor Q3 having its base coupled to the blue drive signal 14B. The current mirror 22 includes transistors Q4 and Q5 having their bases coupled together and to the collector of transistor Q4. The emitters of transistors Q4 and Q5 are coupled to the supply voltage Ncc of the IC 13. The collectors of the red transistor Ql, the green transistor Q2 and the blue transistor Q3 are all coupled to the collector and base of transistor Q4. The emitters of the red transistor Ql, the green transistor Q2 and the blue transistor Q3 are coupled to ground via respective emitter resistors or impedances Rl, R2, R3.

The CRT feedback current replacement circuit 20 further includes current sinking transistor Q10 having a collector tied to the collector of transistor Q5 of the current mirror 22 at node A. At node A, a net current is communicated to the current feedback input terminal 15 of the video processor integrated circuit (IC) 13. The base of transistor Q10 is coupled to ground and the emitter has a resistor or impedance R12 tied to voltage -Vee. As can be appreciated, the collector currents 15 and 110 of transistors Q5 and Q10 supply the net current to node A which defines the cathode feedback replacement current of the CRT feedback current replacement circuit 20. The cathode feedback replacement current is communicated from node A to the current feedback input terminal 15 of the video processor integrated circuit (IC) 13 and is sensed by the AKB sensing circuitry 19. It should be noted that AKB is also known as CRT cutoff stabilization and black level stabilization.

In operation the collector currents of transistors Ql, Q2 and Q3 produced by the sequential reference levels during vertical retrace and mirrored via transistors Q4 and Q5 of current mirror 22, are each only slightly greater than the current sunk by the current sinking transistor Q10. By sinking current through transistor Q10, transistors Ql, Q2 and Q3 remain active at all times with relatively little change in their respective collector currents thereby, providing stable characteristics, especially for their base-emitter voltage. In other words, the output current from transistor Q5 of the current mirror 22 is sunk by transistor Q10. Collector currents from transistors Q5 and Q10 are in the milli- Ampere range. However, the current to the current feedback input terminal 15 is in the micro-Ampere range. Thus, when current 15 is slightly greater than current 110, the loop will stabilize which tends to minimize variations at the base-emitter junctions of the transistors Ql to Q3. In an exemplary embodiment, the CRT "cut-off voltage is for example, +180.0 Volts (in case a first grid of the CRT is connected to ground). However, as is well known, at a voltage of approximately +179.9 V some current is drawn causing CRT illumination. A vertical blanking signal without the reference pulses is applied to the CRT drive amplifiers 16a, 16b, 16c, on line 17, to prevent the bias voltages from being visible on the display 35 and, especially, in display modes where the vertical deflection is underscanned.

Referring still to the schematic diagram of display system 10 of Fig. 2, the output on line 18χ of a respective CRT drive amplifier 16a, 16b, 16c drives a respective red, green and blue CRT's cathode RC, GC and BC, and sinks current of display 35. The AKB output on line 18 of a respective CRT drive amplifier 16a, 16b, 16c can be coupled to ground, as shown in Fig. 2, when the feedback current replacement circuit 20 has to provide the feedback (for example in an operation mode of vertical underscan). If in another operation mode the AKB output is to be used, then this output should be coupled to current feedback input terminal 15 (as will be illustrated in an alternate embodiment shown in Fig. 3). The outputs on line 18₃ of a respective CRT drive amplifier 16a, 16b, 16c drives a respective one of the feedback resistors R7, R8, R9.

Referring now to Fig. 3, an alternate embodiment of the schematic diagram of a display system in accordance of the present invention is illustrated. The display system 100 includes selective switching circuitry 140 for selectively feeding a cathode feedback current from the driving stage 116 or a cathode feedback replacement current from the CRT feedback current replacement circuitry 120 to the current feedback input terminal 115 of the video processor integrated circuit (IC) 130. Since the CRT feedback current replacement circuitry 120 is essentially the same as the CRT feedback current replacement circuitry 20 of Fig. 2, no further discussion will be provided except as related to the selective switching circuitry 140. Referring now to the selective switching circuitry 140, an AKB "on" or "off signal is delivered on line 142 through resistor R23 to the base of transistor Q12 wherein an AKB "on" signal is substantially equal to 5V and the AKB "off signal is substantially equal to 0V. The AKB "on" or "off signal on line 142 is delivered to the base of transistor Ql 1 via resistor R14. The AKB "on" or "off signal on line 142 is also delivered to the base of transistor Q13 on line 144 and the base of transistor Q9 on line 146. The base of transistor Ql 1 is coupled to the base of transistor Q8. The emitters of transistors Ql 1 and Q12 are tied together at node B which receives the second output from the CRT drive amplifiers 116a, 116b, 116c on line 118₂. The current on line 118₂ supplies the cathode feedback current to AKB sensing circuitry 119 via the current feedback input terminal 115. The collector of transistor Q12 is tied to negative voltage -Vee.

Transistors Q8 and Q9 have theirs emitters tied to node C which is coupled to the collector of transistor Q5 of current mirror 122. The collector of transistor Q9 is coupled to the collector of transistor Q10 and to node A. The collector of transistor Q8 is tied to ground. The base of transistor Ql 1 is coupled to the base of transistor Q8 both of which are tied to supply voltage Vcc, on line 152, through resistor RIO at node D. The emitter of transistor Q13 is coupled to the emitter of transistor Q10 and a first terminal of emitter resistor or impedance R12. The other terminal of the resistor R12 is coupled to the negative voltage - Vee. The collector of transistor Q13 is coupled to Vcc on line 150. In this embodiment, the base of transistor Q10 is coupled via resistor Rl 1 to the base of transistor Q8, while resistor Rl 1 is in series with and between resistor RIO and resistor R13. Resistor R13 is coupled to ground. In operation, when the AKB "on" signal is present on line 142, the CRT feedback current replacement circuitry 120 is selectively disabled and the collector of transistor Ql 1 operates to deliver the cathode feedback current from CRT drive amplifiers 116a, 116b, 116c on line 118₂ via feedback line 148 to the current feedback input terminal 115 of the video processor integrated circuit (IC) 130. This cathode feedback current is sensed by the AKB sensing circuitry 119.

On the other hand, when the AKB "off signal is present on line 142, the CRT feedback current replacement circuitry 120 is selectively enabled and the net collector currents from the transistor Q5 via transistor Q9 and transistor Q10 at node A deliver a cathode feedback replacement current to the current feedback input terminal 115. Transistor pair Ql 1 and Q12 as well as transistor pair Q8 and Q9 are alternately biased "on" and "off. Thus, when the AKB "on" signal is present, transistor Qll is on, transistor Q12 is off, transistor Q8 is on, transistor Q9 is off and transistor Q13 turns transistor Q10 off. Therefore, only the resultant cathode feedback current at node B is communicated via line 148 to the current feedback input terminal 115. However, when the AKB "off signal is present, transistor Ql 1 is off, transistor Q12 is on, transistor Q8 is off, and transistor Q9 is on. Also transistor Q10 is on, via transistor Q13. Therefore, the cathode feedback replacement current at node A is the net collector current from the collectors of transistor Q5 via transistor Q9 and transistor Q10. This net collector current is fed to the current feedback input terminal 115. In this embodiment, the CRT feedback current replacement circuitry 120 only needs to be activated during an underscanned mode of operation such as, without limitation, when a 16:9 aspect ratio is displayed in a 4:3 display. Otherwise, during modes other than the underscanned mode, there is no reason to defeat the AKB with an alternate cathode feedback current source. Hence, an external switch (not shown) may be provided on the display 135 to supply the AKB "on'V'off signal on line 142.

Referring still to the schematic diagram of the embodiment of FIG. 3, the output on line 118ι of a respective CRT drive amplifier 116a, 116b, 116c drives a respective red, green and blue CRT's cathode RC, GC and BC and sinks current of display 135. The output on line 118₃ of a respective CRT drive amplifier 16a, 16b, 16c drives a respective one of the feedback resistors R7, R8, R9. The CRT drive amplifiers 116a, 116b, 116c each have a resistor or impedance R4, R5 and R6, respectively, coupled to a first input terminal of a respective CRT drive amplifier 116a, 116b, 116c. Moreover, feedback resistors R7, R8 and R9 are also coupled to the first input terminal of its respective CRT drive amplifier 116a, 116b, 116c. Furthermore, each of the CRT drive amplifiers 116a, 116b, 116c have a second input terminal which receives a vertical blanking signal with our without reference pulses during the reference pulse intervals on line 117 from blanking circuit 117. It should be noted that the vertical blanking signal without the reference pulses is applied on lines 17 and 117 only in the AKB off mode. In the AKB on mode the second input terminal receives blanking signals with reference pulses, which prevent blanking while the output biases are present.

Numerous modifications to and alternative embodiments of the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the structure may be varied substantially without departing from the invention and the exclusive use of all modifications which come within the scope of the appended claims is reserved. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A display system (10; 100) comprising: a video processor (13; 130) producing red, green and blue drive signals (14R, 14G, 14B) having respectively a red output bias, a green output bias and a blue output bias corresponding to a respective black level of the respective drive signals (14R, 14G, 14B); a cathode ray tube (CRT) (35; 135) having red, green and blue cathodes

(RC,GC,BC); a driving stage (16;116) coupled to receive the red, green and blue output biases for driving the red, green and blue cathodes (RC,GC,BC); a CRT feedback current replacement circuit (20; 120) which receives the red, green and blue output biases, which is coupled to the driving stage (16;116) to generate cathode feedback replacement currents corresponding to the respective black levels , and to supply the replacement cathode feedback currents to the video processor (13;130); and blanking circuitry (17;117) coupled to the driving stage (16;116) for selectively generating a vertical blanking signal for blanking outputs of the driving stage (16;116), when the cathode feedback replacement currents are generated.

2. The system (10;100) according to Claim 1, wherein the video processor (13; 130) includes auto kinescope bias (AKB) sensing circuitry (19;119) and the cathode feedback replacement currents are sensed by the AKB sensing circuitry (19;119).

3. The system (10; 100) according to Claim 1, further comprising: switching circuitry (140) for selectively switching between cathode feedback currents from the driving stage (16;116) and the cathode feedback replacement currents from the CRT feedback current replacement circuit (20; 120) to prevent the cathode feedback currents from being visible during a vertical underscanned condition.

4. The system (10; 100) according to Claim 1, wherein the CRT feedback current replacement circuit (20; 120) includes: a current mirror (22; 122) having a first transistor (Q4) and a second transistor

(Q5); a red output bias transistor (Ql) having a base which receives the red output bias, a collector coupled to a collector of the first transistor (Q4) of the current mirror (22; 122) and an emitter coupled to a first resistor (Rl); a green output bias transistor Q2 having a base which receives the green output bias, a collector coupled to the collector of the first transistor (Q4) of the current (22; 122) mirror and an emitter coupled to a second resistor (R2); a blue output bias transistor Q3 having a base which receives the blue output bias, a collector coupled to the collector of the first transistor (Q4) of the current mirror (22; 122) and an emitter coupled to a third resistor (R3); and, a current sinking transistor (Q10) having a base coupled to a bias voltage, an emitter coupled to a fourth resistor (R12) and a collector coupled to a collector of the second transistor (Q5) wherein a net current of a collector current of the current sinking transistor (Q10) and a collector current of the second transistor (Q5) provides the cathode feedback replacement current.

5. The system (10;100) according to Claim 4, further comprising: switching circuitry (140) for selectively switching between cathode feedback currents from the driving stage (16;116) and the cathode feedback replacement currents from the CRT feedback current replacement circuit (20; 120) to prevent the red, green and blue output biases from being visible during a vertical underscanned condition.

6. The system (10; 100) according to Claim 5, wherein the switching circuitry (140) includes: a first transistor pair (Ql 1,Q12) having alternately biased transistors which receive the cathode feedback current from the CRT output driving stage (16;116) and a switching signal, wherein the first transistor pair (Ql 1,Q12) selectively delivers the cathode feedback current to the video processor (13;130); a second transistor pair (Q8,Q9) having alternately biased transistors and which is positioned between the collector of the second transistor (Q5) and the collector of the current sinking transistor (Q10) and which receives the switching signal; and, a switching transistor (Q13) which receives the switching signal and is adapted to selectively turn off the current sinking transistor (Q10).

7. A method of blanking red, green and blue output biases of drive signals on a cathode ray tube (CRT) display (35; 135) comprising the steps of: producing, by a video processor (13;130), the red output bias, the green output bias and the blue output bias; replacing cathode feedback currents to the video processor (13;130) by cathode feedback replacement currents, obtainable via a CRT feedback current replacement circuit (20; 120) from the red, green and blue output biases; creating a vertical blanking signal during a time period when the cathode feedback replacement currents are present and, blanking outputs of a driving stage (16;116) which drives the CRT display (35; 135) with the vertical blanking signal.